Given the lack of experience among the LBs currently on the roster, signing a veteran LB - at least for roster insurance - makes sense to me. Probably an ILB/MLB type, given how many edge-capable players the Pats already have. Such a vet wouldn't need to be a starter, just capable of contributing more than Harris' 181 snaps and 22 total tackles.
Anyway, LB seems to me to be the last "hole" on defense that really needs to be plugged (4th safety and 5th CB notwithstanding) with an experienced NFL player.
I'm not recommending Bowman, but some there's some context that might be worth considering (if folks already haven't).
He missed the final 12 games of 2016 after tearing his Achilles in week-4. When he came back as a starter at the beginning or 2017, the Niners has switched from Jim O'Neil's 3-4 (in which Bowman had become an All-Pro) to Bob Saleh's 4-3. Even though, statistically, Bowman seemed to be performing okay (38 total tackles and a PBU in 5 games), it apparently wasn't a good fit for either Bowman or the team.
The Niners didn't trade Bowman, they released him - reportedly by mutual agreement so that he would have the freedom to find the team that fit him best. The Raiders signed Bowman and he seemed to perform pretty well in their 3-4 - while still working his way back from his Achilles tear - 89 total tackles, 2 PBU, 2.5 sacks. Most of Navarro's production came after the Raiders dumped Ken Norton as their DC, following their loss to the Pats in Mexico City, and had John Pagano take over his duties.
WRT safety, it seems to me that any of Reid, Vaccaro or Boston is pretty certain to be an upgrade over Richards on defense. It also seems likely to me that the Pats will carry only four "real" safeties on the 53-man (NOT including special-teamers like Ebner, regardless of their positional designation on the "official roster").
Given that the Pats safeties are already very experienced, and that two of them turn 31 in Camp, and that this seems to be a rare safety class in which their may be a couple-three good prospects still available in the late-1st an into the 2nd, the Pats could choose to go that way, instead.
Anyway, the "order of events" from this point seems to be...
(1) mandatory mini-camp for veterans (a couple weeks from now)
(2) the Draft
(3) rookie mini-camp (1st or 2nd weekend in May, at team discretion)
(4) all UFAs become SFAs for comp-pick purposes (apparently, the 2nd Monday after the draft)
So, the Pats get an opportunity to see where their returning veterans (including 2017 Psquadders) are with things from a practice perspective (a light re-scouting).
Then they see who they come up with in the draft and where those rookies are with things from a practice perspective.
Then they make decisions about what, if any, SFA veterans they may want to add.